A new process for growing wafer-scale 2D crystals could enable future super-thin electronics.

Since the discovery of the remarkable properties of graphene, scientists have increasingly focused research on the many other two-dimensional materials possible, both those found in nature and those concocted in the lab.

Growing high-quality, crystalline 2D materials at scale, however, has proven a significant challenge.

Researchers led by Joan Redwing, director of the National Science Foundation-sponsored Two-Dimensional Crystal Consortium—Materials Innovation Platform, and professor of materials science and engineering and electrical engineering at Penn State, developed a multistep process to make single crystal, atomically thin films of tungsten diselenide across large-area sapphire substrates.

“Up until now, the majority of 2D devices have been fabricated using small flakes that are exfoliated off of bulk crystals,” Redwing says. “To develop a device-ready technology, you have to be able to make devices on large-area substrates and they have to have good crystal quality.”

The process uses sapphire as the substrate because of its crystalline structure. This structure orients the film growth in a crystal pattern in a process called epitaxy. As small islands of the material form on the substrate and the substrate is heated, the islands spread out across the substrate in a uniform pattern, forming a large-area film without gaps and with very few defects.

The key advance was the use of gas-source chemical vapor deposition to precisely control the island density and rate of spreading to achieve a single layer of the 2D material.

In a related paper, which appears in ACS Nano, a team led by Joshua Robinson, associate professor of materials science and engineering at Penn State, provides the foundational understanding to enable device-ready synthetic 2D semiconductors based on these epitaxial large-area films in future industrial-scale electronics.

“The primary significance of this work is we were able to achieve an understanding of the extrinsic factors that go into having a high-quality 2D material,” Robinson says. “What we found was that even when you grow oriented crystals on a surface, there are other factors that impact the ability to get high electron mobility or fast transistors.”

In particular, they found that there is a strong interaction between the sapphire substrate and the monolayer film, with the substrate dominating the properties. To overcome these challenges, the researchers grew two or three layers, which improved the performance by factors of 20-100 times.

“This is the first real evidence of the effect of the substrate on the transport properties of 2D layers,” Robinson says.

The Dow Chemical Company, the Semiconductor Research Corporation through the Center for Low Energy Systems Technology (LEAST), and DARPA supported this work, in addition to the NSF.

Additional authors of the Nano Letters paper are from Penn State and Rensselaer Polytechnic Institute. Robinson would like to acknowledge Susan K. Fullerton’s group from University of Pittsburgh for valuable contributions to the electrical section of the ACS Nano paper.